1. Field of the Invention
The present invention generally relates to a vehicular radar apparatus for measuring the distance between one's own vehicle, on which the vehicular radar apparatus is mounted, and a preceding vehicle going ahead thereof and the relative velocity (or speed) thereof on the basis of reception electromagnetic waves that are obtained by receiving transmission electromagnetic waves (for example, frequency-modulated continuous waves (FM-CW)) which are transmitted from one's own vehicle and are then reflected by a target object. More particularly, the present invention relates to a vehicular radar apparatus which enhances the accuracy of detection of a preceding vehicle and the reliability in tracking, by performing (arithmetic) operations that use a normal distribution.
2. Description of the Related Art
Hitherto, there have been proposed various vehicular radar apparatuses each for detecting the distance between one's own vehicle and a target object (namely, a preceding vehicle) and the relative velocity on the basis of transmission electromagnetic waves and reception electromagnetic waves.
FIG. 20 is a block diagram schematically illustrating a conventional vehicular radar apparatus.
In the apparatus of FIG. 20, for the purpose of transmitting and receiving electromagnetic waves, a vehicular transmitter/receiver 10 is operative to transmit electromagnetic waves, which are generated by, for example, a laser radar or a millimeter-wave radar, as transmission electromagnetic waves Q1 and to receive waves, which are reflected from a target object A, as reception electromagnetic waves Q2.
Vehicular signal processing means 20 includes an (arithmetic) operation means, and is operative to compute the distance between one's own vehicle and the target object A and the relative velocity R on the basis of data and signals (for instance, distance data Ri and relative velocity data Vi) sent from the transmitter/receiver 10, and to obtain end results and output the obtained results to an external system.
Next, an operation of the conventional vehicular radar apparatus constructed as illustrated in FIG. 20 will be described hereinbelow.
Incidentally, the operation of the conventional apparatus as illustrated in FIG. 20 is disclosed in, for example, Japanese Unexamined Patent Publication No. 6-29540 Official Gazette.
Signal processing unit 20 is operative to estimate the range of values Rs(i+1), which are represented by the distance data at the time when a time period corresponding to a data updating period (or cycle) .DELTA.t has passed, from the following equation (1), which is expressed by using the distance data Ri and the relative velocity data Vi outputted from the transmitter/receiver 10: EQU Rs(i+1)=Ri+Vi.multidot..DELTA.t.+-..alpha. (1)
When the distance data R(i+1) at the time, at which the time period corresponding to the data updating cycle .DELTA.t has passed (since a starting time), is inputted, if the value of this distance data R(i+1) is within the range of values Rs(i+1) of the distance data, it is judged that a detected object is identical with the target object A.
Incidentally, according to the aforesaid equation (1), in the case that the preceding vehicle is located at the distance Ri from one's own vehicle and has the relative velocity Vi, the signal processing unit 20 computes the distance Rs(i+1) by which the preceding vehicle moves with respect to one's own vehicle during the data updating cycle .DELTA.t.
Incidentally, the distance data Ri and the relative velocity data Vi are regarded as data or quantities which change every data updating cycle .DELTA.t. Therefore, in the equation (1), tolerance of .+-..alpha. is set so as to accommodate variation in such data.
For instance, FIG. 21 is a diagram illustrating a signal processing operation of a scan radar. In FIG. 21, Y-axis represents a travelling direction in which the one's own vehicle is caused to go; Pi(Xi, Yi) the current position of the preceding vehicle; and Vxi and Vyi two-dimensional components of the relative velocity Vi(Vxi, Vyi), respectively.
In this case, the current position Pi(Xi, Yi) and the current relative velocity Vi(Vxi, Vyi) of the preceding vehicle correspondingly to the travelling direction of one's own vehicle and to a lateral direction perpendicular to the travelling direction thereof can be obtained.
Incidentally, the tolerances .+-..beta. and .+-..beta. (namely, 2.alpha. and 2.beta.) respectively corresponding to the current position Pi(Xi, Yi) and the current relative velocity Vi(Vxi, Vyi) of the preceding vehicle, which act as data or quantities varies within the data updating cycle .DELTA.t, are established so as to accommodate variations in such data.
Thus, the aforementioned equation (1) is extended or expanded as the following equations (2) and (3), by establishing a window whose sides extending in X- and Y-directions are .+-..alpha. and .+-..beta. in width and length, respectively. EQU Ys(i+1)=Yi+Vyi.multidot..DELTA.t.+-..alpha. (2) EQU Xs(i+1)=Xi+Vxi.multidot..DELTA.t.+-..beta. (3)
Therefore, if the values of X- and Y-components of the position data P(i+1) are within the window corresponding to the equations (2) and (3) when inputting this position data P(i+1) at the time at which the data updating period has passed, it is judged that the detected object is identical with the target object A.
It is, however, known that there is caused considerably large variation in the absolute value of sensor output data (Ri, Vi) obtained in the aforesaid conventional apparatus, because the signal level of a reception signal varies with, for instance, natural conditions, such as temperature and weather, and to running environment.
Therefore, the window range, which is determined by .alpha. and .beta. (see the equations (2) and (3)), to be added to the right side of the aforementioned equation (1) have to be set at large values, respectively. Thus, under some running conditions, there is a fear that another target object is erroneously judged as being identical with the target object A.
Conversely, if the window range is set at a small value, namely, if the window width and length .alpha. and .beta. are set at small values, respectively, so as to prevent the apparatus for making such an erroneous judgement, there is a fear that, even when a detected target object is identical with the object A, the detected target object A is erroneously judged as being a different object.
Namely, each of the window width and length .alpha. and .beta. is a constant value. Thus, in the case that variation in measured data is very small, the window width and length .alpha. and .beta. are too large to discriminate between a target object and a different object. Conversely, in the case that variation in measured data is large, the window width and length .alpha. and .beta. are too small. Consequently, even when a detected target object is identical with the object A, the apparatus cannot judge that the detected object is identical with the target object A.
As above described, in the case of the conventional vehicular radar apparatus, a invariant window range is established. Thus, the conventional vehicular radar apparatus has the following problems. Namely, in the case where the window range is too large, namely, the width and length .alpha. and .beta. are too large, there is a fear that different target objects are erroneously judged as being identical with each other. Conversely, in the case where the window range is small, namely, the width and length .alpha. and .beta. are too small, there is a fear that, even in the case where a detected object is identical with a target object, the detected object is erroneously judged as being different from the target object.
The present invention is accomplished to solve the aforementioned problems of the conventional radar apparatus.